$-\dfrac{9}{3} - \dfrac{7}{2} = {?}$
$ = - {\dfrac{9 \times 2}{3 \times 2}} - {\dfrac{7 \times 3}{2 \times 3}} $ $ = - {\dfrac{18}{6}} - {\dfrac{21}{6}} $ $ = - \dfrac{{18} - {21}}{6} $ $ = -\dfrac{39}{6}$